1. Field of the Invention
The present invention relates to a double cavity toroidal-type continuously variable transmission including a pair of cavities defined by input and output disks with which a power roller can be contacted.
2. Description of the Related Art
Such double cavity toroidal-type continuously variable transmission is known from JP-A-8-159229 and JP-A-2000-46145.
In the double cavity toroidal-type continuously variable transmission disclosed in JP-A-8-159229, an input shaft supporting a pair of continuously variable transmission mechanisms is axially positioned in a casing by a torque-cam-mechanism-side bearing disposed on one of the shaft end portions of the input shaft, and a double helical gear formed in a pair of output disks are threadedly engaged with a double helical gear formed in an output shaft.
Also, in the double cavity toroidal-type continuously variable transmission disclosed in JP-A-2000-46145, an input shaft supporting a pair of continuously variable transmission mechanisms is structured such that the two shaft end portions thereof are axially positioned in a casing.
By the way, a double cavity toroidal-type continuously variable transmission of this kind includes a pair of input disks respectively disposed on the two end portions of an input shaft and a pair of output disks respectively disposed on or formed integral with the central portion of the input shaft. One of the input disks is fixed to the input shaft and the output disks or the other input disk is supported on the input shaft so as to be slidable in the axial direction of the input shaft. By energizing the other input disk toward one input disk using a torque cam mechanism or an oil pressure cylinder, power rollers are respectively pressed and held by and between one input disk and the output disk as well as by and between the other input disk and the output disk to thereby transmit torque between them.
However, in the case where the power rollers are pressed and held by and between the input and output disks as a result of the load of the torque cam mechanism or oil pressure cylinder, the contact portions of these parts and the disks themselves are elastically deformed to a slight degree to thereby change or narrow the distance between the input and output disks and it is known that the sum of the respective deformation quantities reaches the unit of mm. As in the double cavity toroidal-type continuously variable transmission disclosed in JP-A-8-159229, in the case where one end portion of the input shaft is supported on the casing in such a manner that it is immovable in the axial direction, with the above-mentioned change of the distance between input and output disks, the axial-direction position of the central output disk varies to thereby shift the meshing engagement between the double helical gear formed in the output disk and the double helical gear formed in the output shaft.
To solve such a problem, it is necessary that the double helical gear formed in the output disk is divided into two helical gears having opposite tooth groove directions and a thrust bearing is interposed between the two helical gears to thereby allow the two helical gears to rotate with respect to each other. However, this complicates the structure. Also, a large torque is required to allow the two helical gears to rotate with respect to each other and also this torque appears as a transmission torque difference between the two helical gears. Therefore, thrust forces, which are applied to the two helical gears from the double helical gear of the output shaft, are caused to differ from each other. The different thrust forces cause an imbalance between the loads received by the right and left power rollers. This raises a fear that the characteristics of the right and left continuously variable transmission mechanisms can differ from each other.
Also, on receiving the load from the torque cam mechanism or oil pressure cylinder, the input shaft is extended in the axial direction. In this case, as in the double cavity toroidal-type continuously variable transmission disclosed in JP-A-2000-46145, in the case where the two end portions of the input shaft are supported on the casing so as to be immovable in the axial direction, there arises a problem that large loads can be applied to the casing from the bearings disposed on the two end portions of the input shaft.
To solve the above problem, the input shaft may be supported on the casing so as to be movable in the axial direction. In this case, however, unless there is arranged some positioning means, there is a possibility that the input shaft can move arbitrarily.
The present invention eliminates the drawbacks found in the above-mentioned conventional double cavity toroidal-type continuously variable transmission. Accordingly, it is an object of the invention to provide a double cavity toroidal-type continuously variable transmission in which, while positioning an input shaft in the axial direction thereof, not only the load of the input shaft is prevented from being transmitted therefrom to a casing, but also loads to be applied to two power rollers respectively stored in a pair of cavities can be prevented from differing from each other.
In attaining the above object, according to a first aspect of the invention, there is provided a double cavity toroidal-type continuously variable transmission including one of a pair of input disks which is fixed to the axial-direction outer portion of an input shaft, where the other input disk is supported so as to be movable in the axial direction thereof, and an output disk is supported on the axial-direction inner portion of the input shaft so as to be movable in the axial direction thereof; the other input disk is energized toward the one input disk by an energizing unit; power rollers respectively stored in a pair of cavities formed between the mutually opposing input and output disks are contacted with the input and output disks; and, a drive gear formed in the output disk is meshingly engaged with a driven gear formed in output shaft, wherein each of the power rollers is rotated around a trunnion shaft to vary its contact positions with the input and output disks, thereby being able to achieve gear change, and further wherein the input shaft is allowed to shift in the axial direction thereof, the output shaft is prevented from shifting in the axial direction thereof and the mutually meshingly-engageable drive and driven gears are respectively composed of double helical gears which are prevented from rotating with respect to each other.
According to the above structure, even in the case where the axial-direction position of the output disk with respect to one input disk fixed to the input shaft is varied by the energizing force of the energizing unit energizing the other input disk, which is supported on the input shaft so as to be movable in the axial direction thereof, toward one input disk, because the axial-shift allowed input shaft and the axial-shift restricted output shaft are connected together by drive and driven gears respectively composed of double helical gears which are prevented from rotating with respect to each other, the axial-direction shift of the input shaft can be restricted by the output shaft. As a result of this, while positioning the input shaft in the axial direction, not only the load of the input shaft can be prevented from being transmitted therefrom to the casing but also the loads, which are applied from the energizing unit to the power rollers stored in the pair of cavities, can be prevented from differing from each other.
By the way, the first shaft 14 employed in the illustrated embodiment corresponds to the output shaft according to the invention, the double helical gear 23 employed in the illustrated embodiment corresponds to the driven gear according to the invention, the output gear 25 employed in the illustrated embodiment corresponds to the drive gear according to the invention, and the cylinder 71 employed in the illustrated embodiment corresponds to the energizing unit according to the invention, respectively.